On Thu, 15 Jan 2015 16:40:33 +0900 Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
> compound_head() is implemented with assumption that there would be
> race condition when checking tail flag. This assumption is only true
> when we try to access arbitrary positioned struct page.
>
> The situation that virt_to_head_page() is called is different case.
> We call virt_to_head_page() only in the range of allocated pages,
> so there is no race condition on tail flag. In this case, we don't
> need to handle race condition and we can reduce overhead slightly.
> This patch implements compound_head_fast() which is similar with
> compound_head() except tail flag race handling. And then,
> virt_to_head_page() uses this optimized function to improve performance.
>
> I saw 1.8% win in a fast-path loop over kmem_cache_alloc/free,
> (14.063 ns -> 13.810 ns) if target object is on tail page.
>
> ...
>
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -453,6 +453,13 @@ static inline struct page *compound_head(struct page *page)
> return page;
> }
>
> +static inline struct page *compound_head_fast(struct page *page)
> +{
> + if (unlikely(PageTail(page)))
> + return page->first_page;
> + return page;
> +}
Can we please have some code comments which let people know when they
should and shouldn't use compound_head_fast()? I shouldn't have to say
this :(
> /*
> * The atomic page->_mapcount, starts from -1: so that transitions
> * both from it and to it can be tracked, using atomic_inc_and_test
> @@ -531,7 +538,8 @@ static inline void get_page(struct page *page)
> static inline struct page *virt_to_head_page(const void *x)
> {
> struct page *page = virt_to_page(x);
> - return compound_head(page);
> +
> + return compound_head_fast(page);
And perhaps some explanation here as to why virt_to_head_page() can
safely use compound_head_fast(). There's an assumption here that
nobody will be dismantling the compound page while virt_to_head_page()
is in progress, yes? And this assumption also holds for the calling
code, because otherwise the virt_to_head_page() return value is kinda
meaningless.
This is tricky stuff - let's spell it out carefully.
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